High efficiency, compact heat exchangers are needed for applications such as in cryocoolers for providing extremely low temperatures, for example, 80K, which are required for operation of long wavelength infrared sensors. Cooling systems for use in missiles and space equipment must also be rugged enough to withstand the launch environment and must provide space compatibility as required. Another desirable feature for such applications is the capability to operate with a relatively low source pressure. This makes the design of a mechanical compressor much easier and will also increase the operating time if high pressure, stored gas cylinders are used as the gas supply.
One approach to meeting requirements for compact, efficient cooling systems is the perforated-plate heat exchanger. Such heat exchangers are made up of a large number of parallel, perforated plates of high thermal conductivity metal in a stacked array, with gaps between plates being provided by spacers. Gas flows longitudinally through the plates in one direction and counterflows in the opposite direction through separated portions of the plates. Heat transfers laterally across the plates from one stream to the other. Operating principles of this type of heat exchanger are disclosed by R. B. Fleming in Advances in Cryogenic Engineering, Vol. 14, pages 197-204. As stated in this reference, a very large heat-transfer surface area per unit volume can be obtained by use of very small holes; the result is a favorable factor in miniaturization. While the reference discloses the desirability of very small holes, the actual device disclosed employs plates 0.81 mm thick with holes 1.14 mm in diameter and a resulting length-to-diameter ratio in the range of 0.5 to 1.0, the device being designed to operate from room temperature to 80K. In order to improve operation of a compact cryocooler, much smaller holes, in the low micron diameter range, and thinner plates with higher length-to-diameter ratios are needed. Available methods for producing holes, such as by punching as disclosed in this reference, are not effective for the desired hole sizes. In addition to being extremely small, the holes should be uniform in size and shape throughout their length so as to function in the same manner as tubes.
Various types of perforated plates for use in heat exchangers are shown in prior patents. U.S. Pat. No. 4,209,061 discloses perforated plates with large-diameter holes disposed in a stack with the holes slightly offset from one another. U.S. Pat. No. 3,216,484 discloses a cryogenic regenerator having perforated plates with much higher perforated diameters than required for purposes of the present invention. Small holes which make up a very small area of a perforated plate are disclosed in U.S. Pat. No. 3,692,095 for the purpose of providing a slow leak effect.
Compact cryocoolers using other approaches are disclosed in U.S. Pat. Nos. 4,781,033 and 4,489,570. The former of these patents shows layering of fine wire mesh screen to obtain a finely divided heat transferring matrix, and the latter discloses micron-size channels etched in the interfaces of glass plates, but neither of them is concerned with perforated plates.
None of the prior references discloses perforated plates having the required hole structure discussed above or suggests how plates with that structure could be fabricated.